He said, "I shall come tomorrow". (Change into indirect speech).

He said that he would come the following day.

He said he would come the following day.

He said that he would come tomorrow.

He said that he will come the following day.

In indirect speech, we change the original sentence from "I shall come tomorrow" to "he would come the following day".

Because "tomorrow" refers to the day after the day of speaking, which becomes "the following day" in indirect speech.